Traditional coupling structures between screw head and tightening tool are constituted by a diametric groove done in the screw head and a tool end ending in a wedge, with a profile which can be inserted in the groove of the screw head.
There are also known coupling structures called star couplings, in which the head has a cross-shaped recess and the tool end finishes in a cross-shaped point.
In all these cases, in order to attain a threading or unthreading of the screw, the axis of the tool must be aligned with the screw head, which requires good accessibility of the screw. In all cases in which the tool does not affect the screw head in alignment with the axis of said screw, a correct action on the screw is not possible, both for the threading and unthreading thereof, making it impossible to carry out these operations or at least requiring great efforts which also cause the deterioration of the screw head and/or the tool end.
There are also known coupling structures between screw head and tightening tool, based on configurations of curved surfaces which can be complementarily coupled to each other, in which the screw head adopts a curved convex configuration, while the tool end adopts a curved concave configuration, which can be coupled onto the screw head, both of them featuring complementary grooves and recesses. This configuration considerably complicates the manufacturing, both of the screw and of the tool and it also has the risk of breaking the screw head when it is subjected to high tightening torques.